Panoramic and tomographic imaging systems are widely used for attaining images from target areas of humans and animals, and nowadays these systems are also used for taking three-dimensional X-ray photographs from target volumes of humans and animals, whereupon solid-state detectors like radiation sensitive semiconductor sensor, e.g. CCD-sensors or other kind of sensor systems producing digital image data are typically utilized. The principles of the three-dimensional X-ray imaging are disclosed e.g. in U.S. Pat. Nos. 5,359,637 and 5,668,844. For three-dimensional X-ray imaging it is important that the image sensor is in correct position in respect to the X-ray source. The mutual positioning of the radiation source and the irradiation plane for the X-ray photography of the head and neck area, especially for producing three-dimensional X-ray photographs, is disclosed in publication WO-00/57788. For this purpose the imaging apparatus is equipped with an X-ray source and a digital image sensor, which is positioned in the radiation field of the X-ray source of the imaging apparatus, behind the object to be imaged each time, as viewed from the X-ray source is used. The radiation source is positioned on an irradiation plane located at a desired distance from the image sensor and the radiation source is moved on said irradiation plane along an essentially circular path in such a way that the central ray of the X-ray source is directed at the same point during the movement. The sensor is positioned at this point or in its vicinity. In order to image the object from different directions, the X-ray source is optionally stopped at the desired points of the circular path for the duration of exposure in each desired direction of imaging. The distance of the X-ray source from the sensor is measured using measuring means like measuring rod having an adjustable length, which measuring rod is located in the casing and positioned against, for example, a stopper outside the mouth which is connected to the image sensor, followed by directing the X-ray at the sensor mechanically, or by means of a ray of light, or by means of ultrasound.
The main object of the invention is to attain a method and an apparatus for pointing the predetermined position of the target, especially the predetermined distance of the target from the X-ray source. The second object of the invention is to attain such a pointing method and pointing apparatus, which does not impair under any circumstances the quality of the X-ray image. The third object of the invention is to attain such pointing method and pointing apparatus, which is accurate, easy to use and allows controlling the position of the target during X-ray imaging.